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High speed folding amplifiers are often used in flash analog-to-digital converters (ADC) to reduce the number of comparators that are required to perform the conversion. Typically, folding amplifiers are effective when the input signals are allowed to settle and the DC gain of the amplifier is used. However, if the input signal undergoes a sudden and large change from a value close to a first reference level to a point close to a second reference level during the sampling time, the input may not settle prior to the hold phase of the input. In this case the dynamic gain of the amplifier is used to provide the output signal.
A folding amplifier typically has two internal amplifiers that are constantly turned on in a low power state. The dynamic gain of the folding amplifier is typically a function of the switching speed of the switching preamplifier between the various internal amplifiers as the input voltage changes. If the switching by the switching preamplifier between the various internal amplifiers is not accomplished before the hold phase of the input signal begins, the folding amplifier can supply a large transient output signal that is opposite in magnitude to the change in the input signal. This transient output signal will decrease the dynamic gain of the amplifier due to the increase in time it takes the amplifier to compensate and overcome the initial transient. The extra time required to compensate for the output transient signal is inversely proportional to the difference between the input signal and the second reference signal. Therefore a worst case scenario occurs when the input signal changes from a value near the first reference signal to a value near the second reference signal. In this worst case scenario, the time necessary to compensate for the transient condition can severely reduce the useful dynamic gain of the amplifier. This reduces the effectiveness of the folding amplifier in high-speed applications.
Therefore it would be desirable to provide a folding amplifier having a useful dynamic gain that is suitable for high-speed applications.
A folding differential amplifier that includes a switching preamplifier that is used to select between first and second differential amplifiers as a function of an input signal. The switching preamplifier includes first and second outputs that are coupled together by a first shorting switch having an open phase and a closed phase. The first and second outputs are held at a steady state value during the closed phase of the shorting switch and allowed to vary during the open phase of the shorting switch. First and second differential amplifiers each have first and second outputs and the first output of the first differential amplifier is coupled to the second output of the second differential amplifier. Similarly, the second output of the first differential amplifier is coupled to the first output of the second differential amplifier. These cross coupled outputs form first and second amplifier outputs respectively. The first and second amplifier outputs can be coupled together by a second shorting switch having a closed phase and an open phase. The closed phase of the second shorting switch lasting longer than the closed phase of the first shorting switch.
In one aspect, the first and second outputs of the switching preamplifier are coupled to the first and second differential amplifiers via first and second current mirrors. The switching preamplifier programs the current available to each of the first and second differential amplifiers. A first and second bleeder current source can be used to provide a current level that allows the first and second differential amplifiers to be turned off when the current from the first and second differential amplifier plus the first and second bleeder current source respectively, is less than the current in of the first and second current mirrors respectively.
In another aspect a compensating current source and compensation differential amplifier are included with the switching preamplifier and the first and second differential amplifiers described above. The compensating current source provides a decaying current to the compensation differential amplifier during the open phase of the first and second shorting switches. The compensation differential amplifier includes first and second inputs and first and second outputs. The first and second outputs of the compensation differential amplifier are coupled to the first and second amplifier outputs and the first and second inputs of the compensation differential amplifier receive the first and second reference signals respectively.
Additional aspects, features and advantages of the present invention are also described in the following Detailed Description.